DVD resolution is 720x480. 480 lines of resolution.
D2 Digital Tape, the format TNG was mastered to and likely the format WGN has in its tape vault has 486 lines of resolution, 720x486. I was actually just playing a D2 tape for a client yesterday and despite it being composite only, it looks damn good.
There is a lot of misinformation out there on up-scaling. There are very poor and very high quality ways to get it done. How much money is spent generally determines that outcome. Some TV stations cheap out and some spend more money. WGN obviously got something of high quality to broadcast the SD content in their HD channel and we're seeing that with TNG.
A lot of blu-ray players have very high quality scalers built in. Much higher quality than a standard DVD player even with component or HDMI outputs. I'm not aware of any TV set to this day that can match the quality of high-end scalers in some of the premium blu-ray players out there.
of course it will never increase the original resolution, but it can do good enough a job to make it look at least as good as it did on a SD tv, just larger.